Oak wilt was first described in Wisconsin in 1944 where trees were found dying in localized areas. This disease kills young and mature oak Quercus trees and has been found in 22 states. The predominant damage has occurred in the upper Midwest although the range now extends from New York to Texas. The most recent find was in upstate New York during the 2008 season.

Biology

The fungus that causes the disease, Ceratocystis fagacerum, can overwinter as mycelium under the bark of living diseased trees and as fungus mats under the bark on dead trees.

The disease can be spread by sap and bark feeding beetles that feed on the mats of fungal mycelium found under the tree bark primarily in the early spring.

As the fungus mats enlarge and cause the bark to split the emitted odor, which can be described as smelling like "apple cider", attracts insects, usually sap beetles, to feed on the fungal mats and through feeding activity the fungal spores become attached to their bodies.

These beetles then move the fungal disease from diseased trees to healthy trees through feeding and reproduction activities.

Once inside the trees the fungus spreads rapidly inside the water conducting tissues of the xylem causing them to become plugged and nonfunctional.

Trees can also become infected through root grafts between infected and adjacent healthy trees.

Host Trees

This disease attacks all oak species and has been found in 16 native oak species. In general, red oaks (Quercus sect. Lobatae) are more susceptible and frequently infected compared to white oaks (Quercus sect. Quercus). Additional inoculations have demonstrated over 35 native and exotic oak species are susceptible as well as American and European chestnuts (Castanea), chinkapin (Chrysolepis) and tanoaks (Lithocarpus), and several cultivars of apple.

Things to look for

Red oak symptoms can occur as early as May with leaves turning dull green or bronze before wilting and becoming yellow or brown. These symptoms typically affect the leaf tip and margins first as they move inwards towards the midrib and leaf base. Wilted leaves will curl around the leaf midrib. Sometimes these symptoms are mistaken for drought damage.

Dying red oak leaf symptoms often intensify within a few weeks throughout the canopy and the leaves at the ends of the branches often show heavy defoliation.

Leaves will fall in all stages of symptom development. Even apparently healthy green leaves will be shed as trees begin to decline.

Severely diseased red oaks can die within 2 months and most die within a year of visible symptoms.

Recently wilted red oak branches will often show dark streaking under the bark where the fungus has plugged the xylem tissue. Fresh 15-20 cm branch or twig pieces should be placed in plastic bags and transported in an insulated container for examination by trained personnel.

White oaks often have less severe symptoms and may only lose one or two branches a year. Infected white oaks often have discolored annual rings, but seldom develop fungus mats under the bark. Some white oaks may even recover and serve as symptomless reservoirs for this disease.

Management Options

There are no effective methods of using fungicides to save infected oak trees.

Trenching devices and other mechanical methods have been used to cut tree roots to a depth of 60-120 cm to stop the spread of the fungus from infected trees through root graphs to adjacent healthy trees. The sooner the root grafts are broken the better the chances of saving the healthy surrounding trees.

Any trees that have died from oak wilt should be removed before beetle flight the next spring. Any trees that have fungus mats present should also be removed.

Avoid pruning during beetle activity which usually includes April through mid summer.

Trees suspected with oak wilt should be reported and samples collected for identification by state forestry, state department of agriculture or extension personnel. Prompt diagnosis will help prevent disease spread.